Magnetic resonance imaging (“MRI”) has been developed as an imaging technique adapted to obtain both images of anatomical features of human patients as well as some aspects of the functional activities and characteristics of biological tissue. These images and/or functional and/or chemical measurements have medical diagnostic value in determining the state of the health of the tissue examined.
In an MRI process, a patient is typically aligned to place the portion of the patient's anatomy to be examined in the imaging volume of the MRI apparatus. Such an MRI apparatus typically comprises a primary magnet for supplying a constant magnetic field (B0) which, by convention, is along the z-axis and is substantially homogeneous over the imaging volume and secondary magnets that can provide linear magnetic field gradients along each of three principal Cartesian axes in space (generally x, y, and z, or x1, x2 and x3, respectively). A magnetic field gradient (ΔB0/Δx1) refers to the variation of the field along the direction parallel to B0 with respect to each of the three principal Cartesian axes, x1. The apparatus also comprises one or more RF (radio frequency) coils which provide excitation and detection of the MRI signal. Additionally or alternatively, detection coils may be designed into the distal end of a catheter to be inserted into a patient. When such catheters are employed, their proximal ends are connected to the received signal input channel of the magnetic resonance imaging device. The detected signal is transmitted along the length of the catheter from the receiving antenna and/or receiving coil in the distal end to the MRI input channel connected at the proximal end.
The insertion of metallic wires into a body, e.g. catheters and guidewires, while in a magnetic resonance imaging environment, poses potentially deadly hazards to the patient through excessive heating of the wires, e.g. in excess of 74° C. in some studies. M. K. Konings, et. al, in “Catheters and Guidewires in Interventional MRI: Problems and Solutions”, MEDICA MUNDI 45/1 March 2001, list three ways in which conductors may heat up in such environments: 1) eddy currents, 2) induction loops, and 3) resonating RF transverse electromagnetic (TEM) waves along the length of the conductors. They write: “Because of the risks associated with metal guidewires, and catheters with metal conductors, in the MRI environment, there is an urgent need for a non-metallic substitute, both for guidewires and for signal transfer.” They further propose the use of “ . . . a full-glass guidewire with a protective polymer coating . . . . ”
The tracking and placement of a catheter within a body is an important aspect of using catheters in magnetic resonance imaging procedures. Considering the dangers inherent in the use of metallic wires in the magnetic resonance imaging environment, as mentioned above, M. K. Konings, et. al., in their paper “Development of an MR-Safe Tracking Catheter With a Laser-Driven Tip Coil” describe the design of a tracking catheter “ . . . using an optical fiber with a light-diffusing tip segment to transport laser energy through the catheter. This energy is converted to a DC current running through a small coil at the catheter tip. Our method is inherently MR-safe since the use of long conducting wires is avoided.”
From the paper “An Optical System for Wireless Detuning of Parallel Resonant Circuits” by E. Y. Wong, et. al., in the Journal of Magnetic Resonance Imaging 12:632-638 (2000), it is pointed out that typically when a catheter coil is used in magnetic resonance imaging, it is necessary to detune the coil away from the frequency of the magnetic resonance imaging system during the transmission of the magnetic resonance imaging pulse sequence. The authors write “In all MRI experiments in which local coils are used for signal reception, coil detuning is necessary during transmission to prevent high voltages from being induced in the receiver coil and other electronic components including the receiver preamplifier. The potentially high voltages and currents, as well as the induced electric fields, pose a safety hazard for the patient, . . . , and disrupt the desired uniform excitation field generation required for excitation; this may lead to particular localized effects in interventional or intravascular MR imaging in which small coils are used.” This paper further describes the use of a complex design consisting of fiber optic cable and photoresistors to overcome these problems.
By providing a catheter with an MR receiving coil or antenna in the distal end, the coil or antenna can be placed closer to the tissue which is to be imaged or analyzed. Thus the detected signal is less susceptible to radio frequency noise. Additionally, the level of detail that can be resolved (the resolution of the image, spectrum, or other data obtained) is increased by the use of catheter coils.
Thus, it is desirable to provide an apparatus such as a catheter which can be used with a magnetic resonance system for insertion and positioning of an magnetic resonance receiver coil or antenna within a body which is not susceptible to the heating, noise pickup, electrostatic buildup and/or other hazards associated with the use of conductors in a magnetic resonance environment.